iStock/Thinkstock(WASHINGTON) -- Federal authorities Friday urged law enforcement across the country to be alert for possible attacks inside the United States in retaliation for U.S. airstrikes against the Islamic group ISIS, the brutal terrorist group that beheaded American journalist James Foley and has seized vast swaths of territory in Iraq and Syria.

In a joint bulletin issued to local, state and federal law enforcement, the Department of Homeland Security and FBI said that while they are "unaware of any specific, credible threats against the Homeland" and find direct threats from ISIS "not credible," they cannot rule out attacks in the U.S. from sympathizers radicalized by the group's online propaganda.

"[B]ecause of the individualized nature of the radicalization process -- it is difficult to predict triggers that will contribute to [homegrown violent extremists] attempting acts of violence," the bulletin states. Moreover, such lone offenders "present law enforcement with limited opportunities to detect and disrupt plots, which frequently involve simple plotting against targets of opportunity," according to the bulletin.

The group is known by the acronyms ISIS and ISIL and recently changed its name to the Islamic State, claiming it has formed an Islamic caliphate in the areas of Syria and Iraq that it controls.

The beheading of Foley was in retaliation for U.S. air attacks on ISIS fighters, according to a statement from the group. ISIS is holding at least one other American journalist, Steven Soltoff, and is threatening to kill him.